In a heavy skillet over medium heat, sauté onions until transparent. Stir in garlic and cook for 30 seconds. Increase heat to medium-high and add the shredded meat. Fry, stirring frequently for about 3 minutes. Add the sliced chipotle and the chipotle chile sauce and simmer gently for 10 minutes to blend the flavours. Season with salt. Serve on top of tostadas. Serves 4.

Chipotle Chile Sauce
This essential sauce, gently balanced between smokiness of chipotles and sweetness of garlic, is used as the starting point in many Mexican dishes.

Preheat broiler. Line a baking sheet with foil and arrange tomatoes, onion and garlic on baking sheet. Broil until tomatoes, onion and garlic are soft and blackened in spots. Cool. Transfer the ingredients and any juices to a blender; add chipotles and blend until smooth. Transfer to a bowl and season with salt. Makes 1 1/2 cup.